Balloons are often used to dilate constricted tubular body cavities such as arteries, veins, urinary cavities, etc. Basically, a balloon is attached to a tubular catheter body with a facility to inflate such balloon using liquid media. Specifically, balloons are used in various angioplasty procedures such as coronary angioplasty, peripheral angioplasty, dilation of the esophagus and ureter and other urological cavities.
In the recent past, stents have been used to maintain patency of lumens that have been dilated by angioplasty balloons. Stents are medical devices made of metal or plastic material but essentially scaffold the lumen of the body cavity. Several stents, such as one made by Johnson & Johnson of New Jersey, have been used to maintain patency of the lumens so dilated by angioplasty balloons.
It is often necessary to use a balloon not only to place such metal stents in the required location but also to distend the stent so that the stent is well pressed into the tissue of the said body cavity. To press such metal stents to the body cavity requires very high pressure. For stents placed in the coronary artery, normal pressure required is between 15-20 atm. Different stents manufactured by different manufacturers require different pressures to press the stent to the body cavity.
Angioplasty balloons, dilation balloons, in general, are made from polyvinyl chloride, irradiated polyethylene, polyethylene terapthylate, polyamide, polyurethane, and from another host of polymers that can be biaxially oriented to impart strength. It is well-known in the art that a polymeric material that has been formed with a given shape, such as a tube, by melt processing can be subsequently processed to impart higher tensile strength by stretching. By stretching, the molecular structure of the polymer is oriented so that the strength in that direction is higher. In a typical process of making a balloon, a polymeric tube such as nylon of polyethylene terapthylate (PET) is extruded into a tube form first. The tube is subsequently heated to a temperature close to its glass transition temperature at which the tube softens and, by pressurizing the tube from inside, a bubble or balloon is formed. When this operation is carried out in a mold, a predetermined shape of balloon of a given size can be made. Sometimes it is necessary to stretch the tube in a lengthwise direction in order to orient the molecules in the lengthwise direction. This type of operation causes what is known as biaxial orientation. Balloons have been formed as early as 1977 by others for medical devices by biaxial orientation.
Biaxial orientation and the degree of biaxial orientation can be measured by studying the birefringence of the part so formed.
When balloons are formed from PVC, irradiated polyethylene (I-PE), polyethylene terapthylate (PET) or nylon (PA), polyurethane (PU) or others, in order to impart high strength as measured by its burst pressure, it is necessary to as fully as possible biaxially orient the molecules. Most balloons formed by biaxial orientation are, however, distensible. Distensible means that a balloon formed to a certain diameter (d) at a working pressure (p) will slightly grow in its diameter when the pressure is increased. Polyethylene, PVC, PU, and nylon balloons grow well over 10% of its diameter when the pressure is increased by 5-10 atm. On the other hand, PET grows very slightly, often less than 10%.
There are some advantages and disadvantages of distensibility. On one hand, when a balloon is distensible, especially at low pressure, 5-10 atm, it can cause damage to the body cavity or artery as the excessive distension will break, or damage unaffected tissue in the arterial wall. Such damage made during a procedure has been found to be the main cause for restenosis of the artery due to scar tissue formation. Most of the balloons that are distensible, when once expanded beyond their yield point, due to the nonreversibility of the process, are bulky. Polyethylene and nylon balloons, when once expanded, remain quite bulky and do not return to the original thin profile set at the time of manufacture of the balloon.
The other disadvantage, and the most important of all, is the fact that these distensible balloons distend at all pressures. In other words, the distensibility is uniform and happens even at lower pressures. The balloon starts growing its diameter from 2-20 atms in a uniform manner.
Nondistensible balloons, i.e., balloons that are distensible less than 10% of its diameter, such as PET balloons, have the advantage of high tensile strength and have very high burst pressure. These can also be made in very thin wall so that the balloon profile is quite small when folded over the catheter tube. These balloons can be made from material having different molecular weight and molecular weight distribution. Often the molecular weight characteristic is measured by the intrinsic viscosity of the material. Depending on the intrinsic viscosity of the PET polymer used, one will be able to make a very strong or less strong balloon. But in either case, the balloon is nondistensible.
An object of the present invention is to provide a balloon that is strong and that has at least two differing rates of distensibility so that a single balloon can be used effectively for angioplasty as well as to set a stent. In angioplasty, the increased distensibility at higher pressures allows use of a given balloon that is for example 3 mm in an artery that is somewhat larger such as 3.3 mm. The same catheter can be used in the angioplasty and later to deliver and set a stent. Alternatively, the balloon can carry a stent and the angioplasty can be performed by use of a first pressure. By then raising the pressure, the stent is pressed into the tissue to its set position.